Bounce Rates by Traffic Source Medium

SELECT trafficSource.medium AS medium,
       HOUR(SEC_TO_TIMESTAMP(visitStartTime)) AS sessionHour,
       COUNT(totals.bounces)/COUNT(totals.visits) AS bounceRate
FROM [google.com:analytics-bigquery:LondonCycleHelmet.ga_sessions_20130910]
GROUP BY medium, sessionHour
ORDER BY sessionHour, medium